Picosecond time-resolved absorption and emission studies of the singlet excited states of acenaphthylene

Abstract

Radiative and nonradiative processes in acenaphthylene have been examined by a combination of steady-state and picosecond time-resolved emission and transient absorption measurements. Fluorescence from a higher excited state ( S₂) is found to compete with the nonradiative decay of this state. A lifetime of <40 ps is suggested for the S₂ state. The major deactivation pathway of the S₁ state is identified as an efficient S₁→S₀ internal conversion, the rate constant of which is measured to be 2.8×10⁹s^-1. While internal conversion accounts for most of the deactivation of the S₁ state, fluorescence with a very low quantum yield (~6×10^-4) is observed. Transient absorption on the picosecond time scale is assigned to S₁ → S_n absorption. The spectra recorded immediately after excitation are found to have a contribution from vibrationally unrelaxed molecules. A rough estimate for the vibrational relaxation rate is also made.